Arterial diseases and injuries are common and often have severe consequences including death. Imaging arteries serves to detect and characterize arterial disease before these consequences occur as well as defining anatomic features to assist in performing surgery for aneurysmal disease.
A conventional method of arterial imaging includes inserting a catheter into the artery of interest (the artery under study) and injecting radiographic contrast, for example, an iodinated contrast, while taking radiographs of the artery. Radiographs are commonly referred to as X-rays. In this technique, the contrast remains in the arteries for a few seconds during which the arteries appear distinct from both the veins and background tissue in the radiographs.
Although a catheter-based contrast arteriography technique generally provides high quality arterial images, there is a risk of arterial injury or damage by the catheter and its insertion. There may be thrombosis, dissection, embolization, perforation or other injury to the artery itself. Furthermore, such a technique may result in a stroke, loss of a limb, infarction or other injury to the tissue supplied by the artery. In addition, hemorrhage at the catheter insertion or perforation sites may require blood transfusions. Moreover, kidney failure and brain injury may result from the toxic effects of the X-ray contrast.
More recent techniques of arterial imaging are based upon detecting the motion of the blood within the arteries and/or veins. These techniques involve employing magnetic resonance imaging (MRI) to image moving blood distinct from stationary background tissues. (See, e.g., Potchen, et al., eds., xe2x80x9cMagnetic Resonance Angiography/Concepts and Applicationsxe2x80x9d, Mosby, St. Louis, 1993; the text of which is incorporated herein by reference). Such techniques do not necessitate catheter insertion into the artery. These techniques are commonly known as 2D time-of-flight, 3D time-of-flight, MOTSA, magnitude contrast, phase contrast, and spin echo black blood imaging.
With pre-saturation pulses it is possible to primarily image blood flowing in one direction. Since arteries and veins generally flow in opposite directions, these pre-saturation pulses allow preferential visualization of the arteries or the veins. Because these techniques depend upon blood motion, the images are degraded in patients who have arterial diseases which decrease or disturb normal blood flow. Such types of arterial diseases that decrease or disturb normal blood flow include aneurysms, arterial stenoses, arterial occlusions, low cardiac output and others. The resulting lack of normal blood flow is particularly problematic because it is those patients with disturbed blood flow in whom it is most important to acquire good quality arterial images.
A related MRI technique relies on differences in the proton relaxation properties between blood and background tissues. (See, e.g., Marchal, et al., in Potchen, et al., eds., supra, pp. 305-322). This technique does not depend upon steady blood in-flow. Instead, this MRI technique involves directly imaging the arteries after administering a paramagnetic contrast agent. Here, after administering the contrast agent, it is possible to image arteries directly based upon the blood relaxation properties. This technique overcomes many of the flow related problems associated with MRI techniques which depend upon blood motion.
Several experts have performed magnetic resonance arterial imaging using intravenous injection of gadolinium chelates (paramagnetic contrast agents). These experts have reported their results and conclusions. In short, these results have been disappointing and, as a result, the use of gadolinium for imaging arteries has not been adopted or embraced as a viable arterial imaging technique. The images using this technique are difficult to interpret because the gadolinium tends to enhance both the arteries and the veins. Since the arteries and veins are closely intertwined, it is extremely difficult to adequately evaluate the arteries when the veins are visible. Further, the difficulty in interpretation is exacerbated as a result of contrast leakage into the background tissues.
However, MRI has evolved over the past decade to become an accepted technique to image the abdominal aorta and abdominal aortic aneurysms. Advances in magnetic resonance imaging for vascular imaging, known as magnetic resonance angiography, have enabled the additional evaluation of aortic branch vessels. However, limitations in magnetic resonance angiography imaging of the slow, swirling flow within aneurysms, turbulent flow in stenoses, and tortuous iliac arteries have limited the usefulness of these general studies in providing detailed information necessary for preoperative planning. In spite of these limitations, recent developments in gadolinium-enhanced magnetic resonance angiography have overcome several of the imaging problems. (See, e.g., Debatin et al., xe2x80x9cRenal magnetic resonance angiography in the preoperative detection of supernumerary renal arteries in potential kidney donorsxe2x80x9d, Invest. Radiol. 1993;28:882-889; Prince et al., xe2x80x9cDynamic gadolinium-enhanced three-dimensional abdominal MR arteriographyxe2x80x9d, JMRI 1993;3:877-881; and Prince, xe2x80x9cGadolinium-Enhanced MR Aortographyxe2x80x9d, Radiology 1994;191(1):155-64).
There exists a need for an improved method of magnetic resonance angiography which provides an image of the arteries distinct from the veins and which overcomes the limitations of other techniques. Further, there exists a need for an apparatus which facilitates providing an image of the arteries distinct from the veins and which may be implemented in overcoming the limitations of other techniques.
In addition, there exists a need for contrast (e.g., gadolinium) enhanced magnetic resonance angiography of abdominal aortic aneurysms to provide essential and accurate anatomic information for aortic reconstructive surgery devoid of contrast-related renal toxicity or catheterization-related complications attending conventional arteriography.
In one principal aspect, the present invention is a method of imaging an artery of a patient using magnetic resonance imaging. The method includes the steps of detecting an elevated concentration of magnetic resonance contrast agent in the artery and imaging at least a portion of the artery including collecting image data which is representative of the center of k-space after detecting the elevated concentration of magnetic resonance contrast agent in the artery.
In one embodiment of this aspect of the invention, image data which is representative of the center of k-space may be collected when a concentration of the contrast agent in the artery is substantially higher than a concentration of the contrast agent in veins adjacent to the artery. In another embodiment, this image data may be collected when the concentration of the contrast agent in the artery is greater than a predetermined concentration. In yet another embodiment, image data which is representative of the center of k-space may be collected substantially at the beginning of an imaging sequence.
The step of detecting an elevated concentration of magnetic resonance contrast agent in the artery may include measuring a base line signal which is representative of a response of the artery to at least one magnetic resonance radio frequency pulse prior to administering the magnetic resonance contrast agent to the patient.
In one embodiment, the artery may be monitored after administering the contrast agent to the patient to detect the arrival of the contrast agent in the artery. The arrival of the contrast may be indicated by detecting a change in the response of the artery to at least one magnetic resonance radio frequency pulse. This change in the response may be a change in a maximum amplitude of a responsive RF signal or a change in the shape of an envelope of a responsive RF signal.
In another embodiment, image data which is representative of the center of k-space is collected substantially at the beginning of an imaging sequence and while the concentration of the contrast agent in the artery is substantially elevated.
In another principal aspect, the present invention is a method of imaging an artery in a region of interest of a patient using magnetic resonance imaging, comprising the steps of detecting a predetermined concentration of magnetic resonance contrast agent in the artery; and imaging at least a portion of the artery including collecting image data which is representative of the center of k-space after detecting the predetermined concentration of the contrast agent in the artery and while the concentration in the artery is higher than a concentration of the contrast agent in veins adjacent to the artery.
In one embodiment of this aspect of the invention, the technique detects the arrival of the contrast in the artery. In this embodiment, image data which is representative of the center of k-space may be collected substantially at the beginning of a 3D imaging sequence.
In another embodiment, image data which is representative of the center of k-space is collected while the concentration in the artery is substantially higher than a concentration of the contrast agent in veins adjacent to the artery. In this embodiment, the step of detecting magnetic resonance contrast agent in the artery includes detecting a substantially elevated concentration of magnetic resonance contrast agent in the artery and the step of imaging at least a portion of the artery includes collecting image data which is representative of the center of k-space after detecting the substantially elevated concentration of magnetic resonance contrast agent in the artery.
In one embodiment, the magnetic resonance contrast agent is administered to the patient by bolus type injection. Under this circumstance, image data which is representative of the center of k-space is collected substantially at the beginning of a 3D imaging sequence.
In yet another principal aspect, the present invention is an apparatus for imaging an artery in a region of interest of a patient using magnetic resonance imaging. The apparatus includes detecting means for detecting a predetermined concentration of magnetic resonance contrast agent in the artery and, in response thereto, for generating an imaging initiation signal. The apparatus also includes imaging means, coupled to the detecting means, for collecting image data which is representative of the center of k-space in response to the imaging initiation signal.
In one embodiment of this aspect of the invention, the imaging means collects the image data which is representative of the center of k-space substantially at the beginning of a 3D imaging sequence. In another embodiment, the detecting means generates the imaging initiation signal when the concentration of the contrast agent in the artery is substantially elevated.
Finally, in another principal aspect, the present invention is an apparatus for imaging an artery of a patient using magnetic resonance imaging and a magnetic resonance imaging contrast agent, comprising, detecting means for generating an imaging initiation signal in response to detecting the magnetic resonance imaging contrast agent in the artery; and imaging means, coupled to the detecting means, for collecting image data which is representative of the center of k-space in response to the imaging initiation signal.
The imaging means may collect image data which is representative of a periphery of k-space after collecting image data which is representative of the center of k-space.
The present invention overcomes the limitations of other techniques by injecting magnetic resonance contrast agents and collecting image data in such a manner that the contrast level in the arteries is higher than that in surrounding veins and background tissue during collection of image data, including data which is representative of the center of k-space.
A high level of arterial contrast also permits directly imaging the arterial lumen, analogous to conventional arteriography. In short, the present invention is, in comparison or relative to other techniques, a method of magnetic resonance angiography which combines several of the advantages of catheter-based contrast arteriography with the advantages of magnetic resonance imaging while substantially eliminating the disadvantages of each.